We are currently in Trujillo, Peru having trouble with both of our F800GSs. We were driving on the highway and both our bikes felt underpowered. There was also a weird ticking noise on hard throttle openings (like passing a truck). We had recently filled up with gas in a small town so we thought that maybe the gas was bad. We were in the middle of the desert so we put our spare (presumably good) gas into the tanks to try and dilute the bad gas. This seemed to improve the situation a bit but it did not fix it. We got the bikes to a safe location where we are currently staying.

the bikes sound terrible and now stall or have troubles starting. We checked the air filter and in the compartment there was a bit of oil. We cleaned that up and replaced the air filter. the bikes still sound terrible and the oil has returned after taking the bike for a short spin.

As far as we know there is no BMW dealer in Peru and we are worried about driving our bikes anywhere and doing further damage. We are also worried about letting some random mechanic here start stripping the bikes if the problem runs deep. No warning lights are on and both bikes are acting up in similar ways. Any ideas?

Summary of issues:
Underpowered
Oil in air box
difficult to start
stalling
Generally sounding like sh*t

I'm not one to attribute every mechanical malady to bad gas, but it sure sounds like you got a batch if it happened soon after a fill-up and both bikes have similar symptoms. The culprit could be water or low octane fuel (pinging might be what you're hearing on hard throttle) or something other than gasoline came along in your last fill-up. My guess is that your 'fuel' was diluted/contaminated with something cheaper. Since you're near sea level the bikes will want high-octane fuel to prevent pinging.

Your best bet is to empty the tanks and re-fill with the best available fuel. I'd do one bike at a time in case the local gas is worse, but you're in a large city, so good gas shouldn't be hard to find. I can't explain the oil in the air filter, unless it's been regurgitated from the crankcase by the poor running engines. I don't know the GS8 configuration that well, but my former KLRs and my DR-Z pass oil fumes into the airbox to be burned, and if something's amiss I've found oil in there.

Next, find all the auto parts stores you can and look for Seafoam. Their website doesn't list dealers in South America, but Pep Boys, Kragen/O'reilly, Ace Hardware, Checkers, CarQuest and Napa all carry it in the US. Get a couple cans for each bike and dose regularly to keep problems at bay.

We are currently in Trujillo, Peru having trouble with both of our F800GSs.
....
As far as we know there is no BMW dealer in Peru and we are worried about driving our bikes anywhere and doing further damage.

My vote is on low octane. If it were debris or gunking, it would be a hell of a coincidence for it to affect both bikes simultaneously. Water, which sits at the bottom of the tank, gives you an instant (and most likely permanent) engine stall.

Although I love my 800, a bike with a thirst for 95RON (or 91 if you had the dealer adjust it for that) can be a liability in the second and third world, where fuel is often considerably lower than that.

Low octane gives you funny noises under load (pinging / detonation), shit power, and the engine will struggle so much to adjust the ratio right you'll get either massive exhaust overheat or thick palls of smoke out the back (depending which way it goes with mixture to try and compensate).

I'm not one to attribute every mechanical malady to bad gas, but it sure sounds like you got a batch if it happened soon after a fill-up and both bikes have similar symptoms. The culprit could be water or low octane fuel (pinging might be what you're hearing on hard throttle) or something other than gasoline came along in your last fill-up. My guess is that your 'fuel' was diluted/contaminated with something cheaper. Since you're near sea level the bikes will want high-octane fuel to prevent pinging.

Thanks to every one for your input. We'll now drain our tanks, put 97 octane gas in one of the bikes than put a octane booster and a injector's cleaner.

Will report back witht the results. Keep the advice coming in the meantime.

One thing that I forgot to mention is the oil leaking/running down most likely coming from the air-filter chamber's draining hose (usually used for letting fumes scape, i'm guessing?). This is happening to both bikes.

The 'metal' pounding noise coming from the bottom part of the engines, where the letters 'BMW' are (bellow the twin cylinders) on the left side (chain and sprockets side).

Thanks to every one for your input. We'll now drain our tanks, put 97 octane gas in one of the bikes than put a octane booster and a injector's cleaner.

Will report back witht the results. Keep the advice coming in the meantime.

One thing that I forgot to mention is the oil leaking/running down most likely coming from the air-filter chamber's draining hose (usually used for letting fumes scape, i'm guessing?). This is happening to both bikes.

The 'metal' pounding noise coming from the bottom part of the engines, where the letters 'BMW' are (bellow the twin cylinders) on the left side (chain and sprockets side).